This invention relates generally to electrostatic printing, and particularly to electrostatic color printing/plotting using multiple color print stations to produce a full-color image in a single-pass.
Electrostatic printing is accomplished by placing electrostatic charges in the form of the image to be printed on an electrographic media, usually paper. The paper is then exposed to a liquid toner to produce a permanent visible image. In electrostatic color printing, separate images are electrostatically printed on the paper and toner applied, each image typically corresponding to one of four colors: The three colors of yellow, cyan and magenta, and for true black, a fourth color of black.
Heretofore, some electrostatic color printers use a recording head containing an elongate arrangement of styli in combination with a back-plane of one form or another. Electrostatic printing paper is drawn between the recording head and styli while individual ones of the styli are selected and impressed with a voltage potential that, together with the back-plane (usually also impressed with a voltage potential) creates a "dot" of charge on the paper. The image for each color may be thought of as comprising lines or "rasters" of these dots which, when toned combine to form the image of that color. For full color, the yellow, magenta, cyan, and (if used) black images are separately printed, each registered to be relatively superimposed on one another to form the full-color image. Depending upon the final color desired, any particular dot location on the paper may have one or more colors printed thereat. Printing the full-color image has been performed by passing the paper through the print station (which usually has a single electrostatic recorder and multiple toner/dryers--one for each separate image) to print the first image, rewinding the paper and passing the paper through again for each subsequent image.
Among the problems associated with this technique of electrostatic color printing are the time required to complete the total image and maintaining the relative position of each successive image upon the paper relative all the primary images. As the paper is transported forward and backward through the print bed, proper registration of the individual images can be quite difficult. In addition, these problems are exacerbated by changes in the media caused, primarily, by humidity which, in turn, causes the paper to change size, both in the direction of travel and laterally.
Thus, the advantages of a color printer/plotter capable of printing a full-color image in a single-pass are certainly evident to those skilled in this art. The problems of time and registration are reduced, if not obviated, allowing precise full-color images to be printed, one after the other in a single-pass by print stations located in sequence along the transport path. Unfortunately, the drag forces imposed upon the paper by each print station, unless somehow compensated for, would ultimately be too great. Attempting to pull the paper through all print stations would require additional paper strength to prevent tearing. The paper becomes still, less flexible, and more expensive. Alternatively, merely adding additional drives between the print stations does not necessarily solve the problem--and can create other problems. Proper tension across the print head must be maintained within predetermined limits in order for the printing process to occur correctly. Adding extra paper drives can cause paper tension to be unacceptable at some of the print stations.